The API routing in my ASP.NET Core WebAPI project does not work, returns a 404 error

Question

I am using MongoDB to setup and host my database for my project, I've been following the official Microsoft doc on how to set up a Web API for MongoDB. I've done everything just as the doc had but routing isn't working. When I try http://localhost:55536/api/user It returns a 404 error.

The controller:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using StudyAPI.Models;
using StudyAPI.Services;

namespace StudyAPI.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class UserController : ControllerBase
    {
        private readonly UserService _userService;

        public UserController(UserService userService)
        {
            _userService = userService;
        }

        [HttpGet]
        public ActionResult<List<User>> Get() =>
            _userService.Get();

        [HttpGet("{id:length(24)}", Name = "GetUser")]
        public ActionResult<User> Get(string id)
        {
            var user = _userService.Get(id);

            if (user == null)
            {
                return NotFound();
            }

            return user;
        }

        [HttpPost]
        public ActionResult<User> Create(User user)
        {
            _userService.Create(user);

            return CreatedAtRoute("GetUser", new { id = user.Id.ToString() }, user);
        }

        [HttpPut("{id:length(24)}")]
        public IActionResult Update(string id, User userIn)
        {
            var user = _userService.Get(id);

            if (user == null)
            {
                return NotFound();
            }

            _userService.Update(id, userIn);

            return NoContent();
        }

        [HttpDelete("{id:length(24)}")]
        public IActionResult Delete(string id)
        {
            var user = _userService.Get(id);

            if (user == null)
            {
                return NotFound();
            }

            _userService.Remove(user.Id);

            return NoContent();
        }
    }
}

CRUD Operations Service

using MongoDB.Driver;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using StudyAPI.Models;

namespace StudyAPI.Services
{
    public class UserService
    {
        private readonly IMongoCollection<User> _users;

        public UserService(IStudyDBSettings settings)
        {
            var client = new MongoClient(settings.ConnectionString);
            var database = client.GetDatabase(settings.DatabaseName);

            _users = database.GetCollection<User>(settings.UserCollection);
        }

        public List<User> Get() =>
            _users.Find(user => true).ToList();

        public User Get(string id) =>
            _users.Find<User>(user => user.Id == id).FirstOrDefault();

        public User Create(User user)
        {
            _users.InsertOne(user);
            return user;
        }

        public void Update(string id, User userIn) =>
            _users.ReplaceOne(user => user.Id == id, userIn);

        public void Remove(User userIn) =>
            _users.DeleteOne(user => user.Id == userIn.Id);

        public void Remove(string id) =>
            _users.DeleteOne(user => user.Id == id);
    }
}

Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Options;
using StudyAPI.Models;
using StudyAPI.Services;

namespace StudyAPI
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.Configure<StudyDBSettings>(
                Configuration.GetSection(nameof(StudyDBSettings)));

            services.AddSingleton<IStudyDBSettings>(sp =>
                sp.GetRequiredService<IOptions<StudyDBSettings>>().Value);

            services.AddSingleton<UserService>();

            services.AddControllers();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGet("/", async context =>
                {
                    await context.Response.WriteAsync("Hello World!");
                });
            });
        }
    }
}

Running IIS Express works and displays "Hello World!" but aside from that, routing to /api/etc doesn't work.

Answer 1

You should add endpoints.MapControllers(); to the Startup file.
Change the app.UseEndpoints to :

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/", async context =>
    {
        await context.Response.WriteAsync("Hello World!");
    });
    endpoints.MapControllers();
});

I hope you find this helpful.